I'm not specifically familiar with that fork but it is trash now, and probably always has been.

Suspension forks only have a spring on one side, on the other is the damper that controls the movement of the fork so that it does notact like a pogo stick. On decent forks this is accomlished by having a piston travel through oil with the flow of oil directionally restricted by ports in the piston seal.

On really cheap forks the "damper" is really just friction in the system. This tends to wear pretty fast resulting in a pogo stick fork.

It is probably not worth the time and effort to make that bike work well.